A process for producing an unsaturated fatty acid from olefin by way of an unsaturated aldehyde is a typical process of gas-phase catalytic oxidation.
Particular examples thereof include a process of producing (meth)acrylic acid from a staring material such as propylene, propane, isobutylene, t-butyl alcohol or methyl-t-butyl ether (referred to as ‘propylene or the like’, hereinafter) by way of corresponding (meth)acrolein.
In this connection, in the first step of partially oxidizing olefins to unsaturated aldehyde, composite metal oxides containing molybdenum and bismuth are generally used as a catalyst. In the second step of partially oxidizing the unsaturated aldehyde, which is a main product of the first step, to unsaturated fatty acid, composite metal oxides containing molybdenum and vanadium are used as a catalyst.
More particularly, in the first step, propylene or the like is oxidized by oxygen, inert gas for dilution, water steam and a certain amount of a catalyst, so as to produce (meth)acrolein as a main product. Then, in the second step, the (meth) acrolein is oxidized by oxygen, inert gas for dilution, water steam and a certain amount of a catalyst, so as to produce (meth)acrylic acid. The catalyst used in the first step is a Mo—Bi based multinary metal oxide, which oxidizes propylene or the like to produce (meth)acrolein as a main product. Also, some (meth)acrolein is continuously oxidized on the same catalyst to partially produce (meth)acrylic acid. The catalyst used in the second step is a Mo—V based multinary metal oxide, which mainly oxidizes (meth)acrolein of the mixed gas containing the (meth)acrolein produced from the first step to produce (meth)acrylic acid as a main product.
A reactor for performing the aforementioned process is provided either in such a manner that both the two-steps can be performed in one system, or in such a manner that the two steps can be performed in different systems.
As mentioned above, the first-step catalyst involved in gas-phase partial oxidation using propylene or the like as a starting material is the Mo—Bi based multinary metal oxide, with which (meth)acrolein is produced as a main product and 10% or less of (meth)acrylic acid is produced.
As disclosed in JP-A-8-3093, a conventional first-step catalyst is a composite oxide represented by the formula of Moa—Bib—Fec-Ad-Be—Cf-Dg-Ox (wherein Mo, Bi and Fe represent molybdenum, bismuth and iron, respectively; A is nickel and/or cobalt; B is at least one element selected from the group consisting of manganese, zinc, calcium, magnesium, tin and lead; C is at least one element selected from the group consisting of phosphorus, boron, arsenic, Group 6B elements in the Periodic Table, tungsten, antimony and silicon; D is at least one element selected from the group consisting of potassium, rubidium, cesium and thallium; when a=12, 0<b≦10, 0<c≦10, 1≦d≦10, 0≦e≦10, 0≦f≦20, and 0<g≦2; and x is a value defined by the oxidation state of each element). When gas-phase catalytic oxidation of propylene is performed with molecular oxygen by using the first-step catalyst, and by operating the first-step catalyst bed at a temperature of 325° C., acrolein is produced with a yield of 81.3% and acrylic acid is produced with a yield of 11%. In other words, acrylic acid content is low in the reaction product obtained by using the first-step catalyst.
Meanwhile, JP-A-5-293389 discloses a catalyst represented by the formula of MoaBibFecAdXeYfZgSihOi (wherein Mo, Bi, Fe, Si, and O represent molybdenum, bismuth, iron, silicon and oxygen, respectively; A is at least one element selected from the group consisting of cobalt and nickel; X is at least one element selected from the group consisting of magnesium, zinc, manganese, calcium, chrome, niobium, silver, barium, tin, tantalum and lead; Y is at least one element selected from the group consisting of phosphorus, boron, sulfur, selenium, Group 6B elements in the Periodic Table, cerium, tungsten, antimony and titanium; Z is at least one element selected from the group consisting of lithium, sodium, potassium, rubidium, cesium and thallium; and each of a, b, c, d, e, f, g, h and i represents the atomic ratio of each element, with the proviso that when a=12, b=0.01 to 3, c=0.01 to 5, d=1 to 12, e=0 to 6, f=0 to 5, g=0.001 to 1, and h=0 to 20, and i is the oxygen atom number needed to satisfy the atomic valence of each element). When gas-phase catalytic oxidation of propylene is performed by using the above catalyst to produce acrolein and acrylic acid, acrylic acid is produced with a yield of 6.2 mole % under a propylene conversion ratio of 99.1 mole % and an acrolein selectivity of 89.6 mole %. In other words, acrylic acid content is still low in the reaction product obtained by using the first-step catalyst.
Further, the known production process comprising the two-steps of partial oxidation requires a separate reactor or reaction zone in each reaction step, and catalysts having different compositions are provided according to the requirements of each step. That is, hardware (physical operating system) should be controlled and supervised under the optimal reaction conditions, of which complexity gives troubles and difficulties in operation.